Recently, a color image copying method has been made practical, in which an electrostatic latent image of an original image is formed by exposing a photoreceptor to separated light and developed by a single color toner to form a single color image, and several of the thus formed color images are overlapped to form a full color image. Color toners such as a yellow, magenta and cyan toners to be used for such copying method are produced by dispersing a pigment or oil-soluble dye in a binder resin.
In an electrophotographic mage forming method, the image is generally formed according to the following procedure.
Firstly, an electrostatic latent image is formed on a photoreceptor constituted of a photoconductive substance by imagewise exposing the photoreceptor to light corresponding to image information, using various methods. Next, the electrostatic latent image formed on the photoreceptor is developed by a charged toner to form a toner image. The toner image is transferred onto an image recording medium such as typical paper or an intermediate transfer member, and is fixed onto the paper by a thermal fixing apparatus.
In a color image forming method utilizing the electrophotographic method, electrostatic latent images, each corresponding to digital image data separated into each of colors of yellow, magenta, cyan and black, and are each developed by toners of each of the same color as that of the image data. A full color image can be obtained via such a developing process by repeating it four times.
Hitherto, known organic pigments and oil-soluble dyes are used as the colorant for the electrophotographic toner. However, these pigments and the oil-soluble dyes each have specific drawbacks.
For example, though the organic pigments are generally superior to the oil-soluble dyes in resistance to heat and light, the transparency of the image is lowered because the pigments each exist in a state of particles dispersed in the toner so that the covering power is raised, but the dispersing capability of pigment is generally not that high. Therefore, transparency and saturation of the image are reduced, and color reproducibility is deteriorated. Transparency of the toner after fixing is necessary to visually confirm he color of the lowest layer without being covered by the color of the upper layer of a layered toner image. Therefore, high dispersing capability and the coloring capability of the colorant are required to maintain the true color of the original image.
As a method for resolving the drawbacks of common pigment, a method in which a flushing method is applied to form primary particles in the submicron order, without producing secondary particles, which improve transparency, and a method in which the pigment particles are covered by a binder resin and a shell resin layer to improve the charging capability, fixing capability and image uniformity of the pigment are proposed (please refer, for example, to Patent Documents 1 and 2).
However, sufficient transparency is difficult to obtain even when the image is printed out by the use of the pigment toners proposed in the above cited documents.
In principle all colors can be reproduced by the subtractive mixture of the three colors of yellow, magenta and cyan. However, many problems exist for reproducing the exact color of the original image because color reproducibility and chromaticity of the reproduced image is in practice deteriorated according to the spectral property of the pigment dispersed in thermoplastic resin, and color mixing adaptability of the toners when the toners are superposed.
On the other hand, toners employing an oil-soluble dye or a mixture of pigment and oil-soluble dye are disclosed (please refer, for example, to Patent Documents 3 and 4).
Oil-soluble dyes are generally superior in transparency and saturation since the oil-soluble dye exists in a dissolved state in the binder resin of the toner, however such oil-soluble dye is much inferior to pigment in resistance to heat and light. Regarding heat resistance, some problems are that the image density is lowered due to decomposition of the oil-soluble dye and contamination in the apparatus tends to be caused by sublimation of the oil-soluble dye during fixing the toner image by heated rollers, and offset is caused by silicone oil, in which the oil-soluble dye is dissolved and adhered onto the heated rollers.
For resolving such problems, one method in which a magenta toner containing a specific anthraquinone type dye or a chelated dye is used to enhance compatibility of light resistance and sublimation with color reproducibility and a capsuled toner constituted by a core containing a polymer resin and a color dye, and a polymer covering the core are proposed (please refer, for example, to Patent Documents 5 -7).
However, sufficient heat (sublimation) resistance and light resistance can hardly ever be obtained by toner using dye even when the image is printed out by using the above cited toners. Thus, development of a toner which more satisfies the above conditions is sought.
As to the method for producing the electrophotographic toner containing common pigment, toner particles obtained by a usual crushing method and toner particles obtained by a wet method employing a polymerization process are known. In the crushing method (or pulverizing method), the targeted toner is produced via processes of mixing the oil-soluble dye and resin, kneading, crushing and classifying. In the case of the polymerization method, for example, a polyester polymerized toner has been proposed which is produced via an interface polymerization method by dissolving or dispersing a pre-polymer, pigment and wax in a solvent and emulsified in an aqueous medium, subjected to interface polymerization and then the solvent is removed (please refer, for example, to Patent Document 8).
Further, as to a polymerized toner, proposed is a method to prepare undefined shape toner particles with association or salting out/fusion of resin particles and colorant particles as needed (please refer, for example, to Patent Document 9). However, the colored particles exhibit an average particle diameter of 112 nm, which is unsatisfactory since dispersion is conducted employing a Clearmix Dissolver. Further, as to the polymerized toner using a nickel chelating oil-soluble dye (please refer, for example, to Patent Document 10), it has been proven after detailed study by the inventors of this invention that corrugated broadening and an increase of the deterioration rate in light resistance under high humidity conditions.
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 9-26673
Patent Document 2: JP-A 11-160914
Patent Document 3: JP-A 5-11504
Patent Document 4: JP-A 5-34980
Patent Document 5: JP-A 8-69128
Patent Document 6: JP-A 10-20559
Patent Document 7: JP-A 5-72792
Patent Document 8: JP-A 2002-169336
Patent Document 9: JP-A 2002-221823
Patent Document 10: JP-A 2006-106561